In a digital television bi-directional service, a user may use the Video On Demand (VoD) service. A structure of a cable digital television system is as shown in FIG. 1, and the process of requesting a program in the system is described in the following: A user visits an Electronic Program Guide (EPG) server 1 by using a Set Top Box (STB) 5, browses an EPG, and selects a program. Middleware 2 controls a VOD server 3 according to VOD information to output a code stream of the program at an inherent code rate of the code stream of the program, and sends program description information such as a transmission frequency point of the code stream and a Service-ID to the STB 5. An Internet Protocol (IP) Quadrature Amplitude Modulator (IPQAM) 4 multiplexes and outputs the received code stream of the program. The STB 5 decodes and plays the corresponding program or audio and video stream according to the received program description information.
The IPQAM is a device configured to perform up-conversion on the code stream, in which the IPQAM receives a Transport Steam (hereafter referred to as a TS stream) input from an IP port, multiplexes the TS stream, and then outputs a Radio Frequency (RF) signal to the STB, so that the code stream is transmitted in a Hybrid Fiber Coax (HFC) network.
In the VOD service, the IPQAM receives the code stream of the program from a corresponding port, then multiplexes the code stream, converts the code stream into an RF signal, and outputs the RF signal. The cable digital system serves each user individually, so that the IPQAM receives the code stream from one port, converts the code stream into the RF, and outputs the RF for only one user every time. A bandwidth provided by one frequency point is fixed and the bandwidth is represented by Q, for example, currently the bandwidth widely used in China is 38 M. The inherent code rates of the programs are the same and represented by X, so that the number of the programs that can be transmitted by one frequency point is Q/X.
The capability of outputting the code stream of the VOD server also serves one user individually. For example, the VOD server may output 1000 streams at the same time, and may serve 1000 users at the same time. When the VOD server serves the 1000 users, if excess users request the service, the VOD server cannot satisfy the excess requests.
In conclusion, within the period of playing the program, the user monopolizes resources of the IPQAM and the VOD server. Therefore, when many users request a program at the same time, a head-end system needs to provide the same number of code streams as the users requesting the program at the same time. In addition, the larger the number of the users requesting the program is, the larger the number of the streams output by the system is required, and therefore the system requires higher performance. As a result, costs are high. The head-end system is a system on an operator side. Referring to the structure shown in FIG. 1, the head-end system includes an EPG server, a VOD server, an IPQAM, and middleware.
Because most of the users watch the television programs in a concentrated period, for example, between 19:00 and 20:00, the number of the users that watch the program in other time periods is small. Therefore, if other time periods are utilized without greatly increasing the costs, the costs of the entire system are reduced.
During the procedure for implementing the present invention, the inventors find that the prior art has at least the following problems as follows: In the concentrated time periods of watching the television programs by the users, the number of the code streams of the program to be output by the system is large, and therefore the burden is heavy, so that the system requires higher performance, and therefore the system costs are high. On the other hand, in the dispersed time periods of watching the television programs by the users, the frequency point resource is seriously wasted due to idle states.